When you create a new document configuration (or when you modify an existing configuration) under Settings > Document and email configuration, your existing documents won't automatically be updated.
And that's logic, because it's too risky to automatically modify a bunch of documents that may already have been sent to clients.
To apply a new document configuration to a document, you have to regenerate the PDF file (= click again on Actions > Generate PDF).
If you added Terms & Conditions to a document configuration after having already created documents, you thus have to regenerate them to add your Terms & Conditions on these documents. Don't forget, in this particular case, to click on Reset in the Terms part (at the bottom of the document) and on Update terms and notes.